1. Field of the Invention
This invention relates to magnetoresistive (MR) heads for use in the playback of magnetically recorded signals. More particularly, the invention relates to MR heads of the unshielded type, often referred to as UMR heads, wherein an elongated thin film is brought into direct contact with a recording medium and, in response to signals recorded in the medium, signal current flowing the length of the film is caused to vary.
2. Description Relative to the Prior Art
In the interest of describing the state of the art, as well as the PROBLEM addressed by the invention, reference should first be had to the MR head of FIG. 1: A sense current i is passed through an MR sense film 10 via contacts 12, 14. In response to signal information magnetically recorded at A of a recording track 16, the resistance of the film varies to cause the current i to vary accordingly. As is fairly conventional, a magnetic layer 18, in proximity to the MR sense film 10, magnetically biases the magnetic moment M of the film 10: i.e., the sense current i sets up a magnetic field which, in traversing the layer 18, biases the ordinarily longitudinal moment to an orientation, as depicted.
To preclude a recorded signal field, say, at a track location B from corrupting the playback of signal information from the track location A, it is customary to shield (20, 22) the MR film 10.
FIG. 2 depicts schematically various typical domains which may be associated with the film 10 of FIG. 1. In response to a recorded signal field M.sub.s, the domains will move, expand, and contract; and, as is known, such domain variations are manifested as signal noise, viz. Barkhausen noise.
The elimination of Barkhausen noise may be achieved, as indicated in FIG. 3, by significantly extending the length of an MR film 10', whereby only a single domain may intrinsically exist in the film 10'. This is a valid solution to the Barkhausen noise problem, but it is restricted, realistically, to head configurations wherein only a single record track 16' is read at any given time. This may be appreciated from a quick examination of FIG. 4 which shows schematically the difficulty of magnetoresistively reading simultaneously a number of closely spaced tracks 16a, b . . . , given that "long" MR sense elements 10a, b . . . are employed.
U.S. Pat. No. 4,547,824 discusses another technique for ridding an MR sense film of a plurality of magnetic domains, viz. by applying an easy axis bias field along the longitudinal axis of the film, whereby domain walls are swept from the film. Thus, with but a single domain existing in the film, Barkhausen domain movement is intrinsically impossible. Such an easy axis bias field is complementary to a (second) hard axis field usually applied in the plane of the field perpendicular to the longitudinal axis of the film.
With a shielded MR head, the application of a field that is along the easy axis of its sense film is not a readily apparent and practicable thing to achieve. Because, however, of known signal processing techniques which dispose of problems associated with the undesired influence of "upstream" fields (B, FIG. 1) vis-a-vis of UMR head, the viability of a practical UMR has been recognized.